This invention pertains generally to the field of micro-electro-mechanical systems, to actuators for such systems, and particularly to electrothermal actuators and rectilinear transmissions for such actuators.
Electrothermal actuators have several desirable characteristics for use in micro-electro-mechanical systems (MEMS), including high output forces, low actuation voltages, and electrically conductive structural materials. Simple and cascaded bent-beam electrothermal actuators have been used for rectilinear motion parallel to the substrate plane. See, L. Que, et al., xe2x80x9cBent-Beam Electro-Thermal Actuators for High Force Applications,xe2x80x9d IEEE Intl. Conf. on Micro Electro Mechanical Systems (MEMS ""99), Orlando, Fla., January 1999; J. S. Park, et al., xe2x80x9cLong Throw and Rotary Output Electro-Thermal Actuators Based on Bent-Beam Suspensions,xe2x80x9d IEEE Intl. Conf. on Micro Electro Mechanical Systems (MEMS ""00), Miyazaki, Japan, January 2000. Such devices have produced maximum displacements and maximum blocking forces in the range of 8 xcexcm and 2.5 mN, respectively, for a silicon device of 2,000 xcexcm length, 6 xcexcm width, 4.5 xcexcm thickness, and 0.2 radian bending angle, operating at 400xc2x0 C. For many applications, a longer displacement is necessary, a smaller force is adequate, and the operating speed of incremental mechanisms such as inchworms is insufficient.
Compliant mechanisms have been proposed as transmission systems for MEMS applications. Compliant mechanisms are structures that deform elastically to transmit a force or displacement. See, S. Kota, et al, xe2x80x9cTailoring Unconventional Actuators Using Compliant Transmissions: Design Methods and Applications,xe2x80x9d IEEE/ASME Trans. on Mechatronics, Vol. 4, No. 4, 1999, pp. 396-408. Advantages of compliant mechanisms include the elimination of the friction, wear, and backlash that are common in conventional transmission systems that have mechanical joints. Because of their monolithic construction, compliant mechanisms are also easier to fabricate at the micro-scale level, making them attractive for MEMS applications. Compliant transmissions have been proposed for utilization with electrothermal actuators. See, T. Moulten, et al., xe2x80x9cMicromechanical Devices with Embedded Electro-Thermal-Compliant Actuation,xe2x80x9d MEMSxe2x80x94Vol. 1, ASME International Mechanical Engineering Conference and Exposition, MEMS, Nashville, Tenn., November 1999, pp. 553-560; J. Jonsmann, et al., xe2x80x9cCompliant Thermal Microactuators,xe2x80x9d Sensors and Actuators (A), Vol. 76, 1999, pp. 463-469. A significant issue in the construction of useful electrothermal actuator systems with compliant mechanisms is the force-displacement trade-off that is faced where the application requires much larger displacements than are typically available from electrothermal actuators. For example, for applications such as optical switching in which an incoming optical fiber is moved between one of two output fibers, the displacement requirement is on the order of 100 xcexcm or more, which is 10-20 times the output displacement of typical electrothermal actuators.
The micromechanical electrothermal actuation apparatus of the invention is well suited to micromechanical applications in which rectilinear displacements on the order of 100 xcexcm or more are required, such as in optical fiber switches. The actuation apparatus is compact and may be formed to occupy an area a few millimeters or less on a side, with relatively low voltage power sources required.
The actuation apparatus of the invention includes a substrate having a surface, with an actuator mounted on the substrate having two output beams and responsive to electrical power supplied thereto to drive the two output beams inwardly or outwardly in opposite directions. The actuator may comprise an electrothermal actuator. A micro-transmission is also mounted on the substrate and comprises a mesh of compliant structural beam elements connected together at nodes. The micro-transmission has two input nodes, each of which is attached to one of the two output beams of the actuator, which are displaced as the output beams are driven inwardly or outwardly. The micro-transmission couples the force from the two output beams and transmits the displacement of the output beams to an output node with an amplification of the output node displacement with respect to the displacement of the input nodes. A very large displacement amplification factor, in the range of 10 to 20 or greater, may be provided by the micro-transmission. Because the micro-transmission receives displacements from two output beams of the actuator and couples the force from these two beams together, the force applied to the output node of the transmission is greater than would be available from an actuator providing displacement of a single output beam.
An exemplary actuator that may be utilized in the invention is an electrothermal actuator comprising two anchor mounts mounted on the substrate spaced from each other and two pairs of beam elements, with each pair of beam elements joined at a vertex to form an inwardly or outwardly bent beam extending between the two anchor mounts. The actuator output beams are attached to the vertices. Current can be passed through the beam elements between the anchor mounts to cause heating and expansion of the beam elements, causing each vertex joining each of the pairs of beam elements to be displaced inwardly or outwardly, thereby displacing the output beams inwardly or outwardly.
The beam elements of the micro-transmission may include a symmetrical transmission structure including, for each output beam of the actuator, three beam elements forming a triangle one vertex of which is connected to an output beam of the actuator, an anchor mounted to the substrate, a beam element connected from the anchor to join a second vertex of the triangle, and a beam element joined to the third vertex of the triangle and extending to a connection at the output node of the micro-transmission. The length, thickness and orientation of the beam elements in the micro-transmission are preferably optimized to provide a selected amplification of displacement from the input nodes to the output node. The beam elements in the actuator and in the micro-transmission may be formed of various micromechanical materials, including crystalline silicon and electroplated metal such as nickel. Preferably, the beam elements have widths of 50 xcexcm or less, thickness of 500 xcexcm or less, and with the overall area of the actuation apparatus on the substrate less than one cm2.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.